Headerless tables don't work in GitHub Markdown output

[waldyrious]

GitHub Markdown does not support headerless tables. When the "Header location = None" is selected, the resulting table is

| lorem    | ipsum | dolor       |
| sit      | amet  | consectitur |
| adipisci | elit  |             |

which results in this:

| lorem | ipsum | dolor |
| sit | amet | consectitur |
| adipisci | elit | |

Instead, it should be:

|          |       |             |
|----------|-------|-------------|
| lorem    | ipsum | dolor       |
| sit      | amet  | consectitur |
| adipisci | elit  |             |

which results in:

lorem	ipsum	dolor
sit	amet	consectitur
adipisci	elit

See mixmark-io/turndown-plugin-gfm#10 and https://stackoverflow.com/questions/17536216/create-a-table-without-a-header-in-markdown for similar discussions elsewhere (with the same suggested wokaround).